L13 & 14 - Episodic Memory Flashcards
Medial Temporal Lobe Memory System
How did HM acquire the deficit?
Surgery removed parts of the hippocampus to deal with epileptic fits
Medial Temporal Lobe Memory System
What was HM’s memory deficit?
- Spared short-term memory (e.g. repeating a digit sequence)
- Profound anterograde amnesia = inability to learn new information
- Limited retrograde amnesia = inability to remember memories acquired prior to surgery
- More distant past spared
- MTL crucial for acquiring new long-term memories and plays a time-limited role in their storage or retrieval
- More distant past spared
- Memories for personally experienced events impaired
- Semantic memory (e.g. facts, general knowledge) spared
- Could also acquire some new semantic knowledge
- Procedural (skill) memory spared
- Mirror-drawing
- Motor sequence learning
- Priming (a form of implicit memory) spared
- Not all types of memories depend on the MTL
Themedial temporal lobe(MTL) is a brain region that is known to mediate functions of episodic memory and learning (Squire et al 2004).
Medial Temporal Lobe Memory System
How is memory organised?
- Refer to STM as Iconic Memory (very brief) and Working Memory (Limited in capacity)
- Important structures involved in memory
What is episodic vs semantic memory?
- Episodic memory contains contextual information about where and when some event (what) took place
- Memory for semantic facts is generally free of experiential context -Includes personal semantics, ie. facts about oneself
- Autonoetic vs noetic consciousness (Tulving, 1985)
- Autonoetic consciousness - Reliving a memory as if you are part of it, self-referential (linked to episodic)
- Noetic consciousness - Knowing facts but not placing yourself in an episode (linked to semantic)
What areas of the brain were damaged in HM’s surgery?
- H.M.’s surgical resection:
- Temporal pole
- Amygdaloid complex
- Entorhinal cortex (sits on top of the hippocampus - wraps around it)
- Most of the hippocampal complex
Perirhinal cortex vs Parahippocampal cortex and hippocampus
What is the medial temporal lobe (MTL) declarative memory system?
- Similar in humans and monkeys, but different in rats
- But connectivity is the same in all species
- Entorhinal Cortex feeds information to hippocampus and receives it - very tight connectivity between them
- Perirhinal cortex and Parahippocampal receive projections from entorhinal cortex
Perirhinal cortex vs Parahippocampal cortex and hippocampus
What is the Perirhinal cortex (PRC)?
- Familiarity-based item recognition
- Lesions lead to impairments in recognising objects, words, faces
- Activations in PRC during learning correlate with later ratings of familiarity (Know from fMRI experiments)
Perirhinal cortex vs Parahippocampal cortex and hippocampus
What research is there for the PRC?
- Ranganath et al. (2004) - fMRI study looking at PRC activation - Looking at BOLD response during encoding of items and then they are tested on their recognition memory (if the objects were previously shown). Gave a rating of their confidence.
- More activity in PRC in items PPT measured as confidently familiar
- Davachi et al. (2003) - Do people remember the context they remembered the item
- During learning, activity is high with the context and without the context
- Implies PRC is not encoding information about the context, only the items
Perirhinal cortex vs Parahippocampal cortex and hippocampus
What is the PRC useful for in non-memory processes?
- Learning associations about and between objects
- Might share features but in different configurations (of how they are arranged)
- Object perception
- Connectivity wise it projects mostly to the anterior parts of the hippocampus
Perirhinal cortex vs Parahippocampal cortex and hippocampus
What is the Parahippocampal cortex (PHC) important for?
- Recollection of context information
- Activations in PHC during learning and retrieval are high when context is successfully remembered
- Ranganath et al (2004)
- Asked to recall items and what list they were from
- PHC important for bringing back to mind the source and the context
- Activations similar during learning and retrieval
- Autobiographical memory
- Spatial memory and navigation (relies upon remembering things in certain places)
- Scene perception (Parahippocampal place area)
- Connectivity → Projects mostly to posterior parts of the hippocampus
- Similar activation pattern to the hippocampus during recollection tasks
- RSC area also important in this network (next lecture)
Long-term Memory Consolidation
What is standard consolidation theory?
Dates back to H.M., various proponents (e.g. Larry Squire)
- Information initially represented in different parts of neocortex is bound together into a memory trace by the MTL (and possibly the diencephalon)
- There is a short period of cohesion or synaptic consolidation lasting between seconds and minutes
- This is followed by a prolonged period (years) of long-term (or system) consolidation
- Repeated activation - Makes the memory stable
- Sleep - Needs to occur during sleep - When memory is moved out of medial temporal lobe structures
- The MTL is initially necessary for reactivation of these memory traces, but the memory trace is eventually fully consolidated in the neocortex and becomes independent of the MTL
- There is a temporal gradient in retrograde memory
- Events and facts acquired more recently are more severely affected by damage to those structures
- More distant past memories better preserved as they’ve moved out of the MTL
- Applies equally to all types of declarative memory:
- Episodic memory of personally experienced events
- Semantic memory of facts about the world and public events
- (but not “general semantics” – i.e. conceptual knowledge, words)
Long-term Memory Consolidation
What are some challenges to standard consolidation theory?
- Some patients (including H.M., in retrospect) do not show a clear temporal gradient in retrograde memory
- Retrograde amnesia can be very extensive (decades)
- Even when old memories are retained, careful testing shows that they are more schematic and lacking in rich detail
- Not the level of detail needed to “re-experience” the event
- Dissociations in temporal gradients for different types of memory
- Personally experienced episodic memories (autobiographical memory) often impaired for all of the patient’s past
- Semantic memories more likely to show a temporal gradient, with better recall of older memories
Long-Term Memory Consolidation
What is the multiple trace theory?
- Nadel & Moscovitch (1997)
- Hippocampal complex automatically encodes all aspects of experience, represented at different neuronal sites (e.g. visual parts in visual part of the brain)
- Acts as an index or pointer to neurons that represent the information and binds the whole episode into a memory trace (from all the different areas)
- Provides the spatial context for the memory (dependent on hippocampal complex)
- Re-activation of memory occurs in different neuronal and experiential context
- Results in a new memory trace, because hippocampus automatically encodes all attended information
- Traces share some or all of the information about the initial episode
- Multiple traces facilitate extraction of factual information common to all, which becomes independent of episodic context (e.g. the gist becomes separate)
- Older memories have more traces, so are more resistant to damage and have multiple retrieval routes
Leads to a temporal gradient for semantic information for facts as more recent information won’t have as many routes so is more vulnerable to damage
Episodic (spatial and temporal) context continues to be dependent on the hippocampal complex for as long as the memory trace is viable
No temporal gradient for autobiographical episodic memories because it is not based in the hippocampal complex
Long-Term Memory Consolidation
What are the imaging studies supporting multiple trace theory?
Ryan et al (2001) found no difference in patterns of hippocampal activation for recent (< 4 years) and remote (>20 years) memories.
Gilboa et al (2004) found that hippocampal activity was correlated with the vividness of re-experiencing the memory, rather than age of memory per se
Extended Diencephalon-MTL memory system
What is the diencephalon-MTL system?
Contain different nuclei in the thalamus
- Hippocampal complex projects to the anterior thalamus and the mammillary bodies via the fornix
- Anterior thalamus projects back to the hippocampus and amygdala mostly via the cingulum bundle
Extended Diencephalon-MTL memory system
What is the extended hippocampal-diencephalic system & its use?
- Focus on the connection between the anterior thalamic, fornix and hippocampus
- Aggleton & Brown (1999) proposed that the extended hippocampal-diencephalic system is involved in efficient encoding and, therefore, normal recollection of episodic memories
- Damage to any part of this system produces similar memory impairments as networks are very tight
Extended Diencephalon-MTL memory system
What is the perirhinal-diencephalic system & its function?
- A separate system to the extended hippocampal-diencephalic system & involves perirhinal cortex in projections
- Does not involve hippocampus
- Involved in familiarity-based item recognition
Retrosplenial cortex, episodic memory & spatial navigation
What is the Retrosplenial cortex?
- RSC comprises Brodmann areas 29 and 30 (just behind the corpus callousm)
- Strong anatomical connections with a range of memory structures (black arrows represent strong connections)
What is the role of the ‘core network’ of regions involved in episodic memory?
- Constructive episodic simulation (Addis & Schacter)
- Scene construction (Hassabis & Maguire)
- Self-projection and mental time travel (Buckner & Carroll)
- Creating ‘situational models’ (Ranganath & Ritchey)
- Anterior temporal system (AT)
- Item recognition and assessing the significance of entities
- Noetic consciousness - Aware of facts, concepts and words
- More lateral part of temporal
- Posterior medial system (PM)
- Episodic/autobiographical memory and situational models
- Autonoetic consciousness - Placing yourself in situations
- These systems are independent
- Posterior system is vulnerable to Alzheimer’s whereas anterior is vulnerable to frontal temporal dementia
